Jodi's email to the already dead Travis is telling.
She talks about Cancun--strange, since she insists she DID NOT CARE ABOUT CANCUN & the fact he wasn't taking her. She refers to the trip in a carefree way in that email--which makes me believe she felt just the opposite. Esp. since she incessantly talks about other trips she thought the two of them were going to make together--Oregon, Crater Lake, etc.
She also mentions crashing at his house while he's in Cancun with Mimi. And sleeping in his bed. That is psycho. She is desperately trying to control the situation with being left out of Cancun by setting up camp in his house & being there the second he returns.
I think that she wanted to go to Cancun very badly, and if she couldn't, she wanted to stay in his home and bed until he returned. I think Travis said no to both, and this enraged her. He probably told her no on a phone call prompting her to set into action all the premeditation--gas cans, hair, turned off cell phone, etc.
She constantly talks of being "officially" broken up with Travis--but they still met almost daily and had sex. This allows her to control his relationships with other women since even if he's dating someone, Jodi is taking care of the sexual aspect herself.
She feels superior doing this--she feels that Travis sees her as a sex goddess, so they will always be connected by this sex. She also tries to control who he travels with--she's decided that she will be his only travel companion & she refers to that '1000 places' book very often. On top of that, she uses "friendship" to justify living in his home and sleeping in his bed. Another control thing.
I think her motive for the killing was the realization that Travis was cutting her out of all these activities. (Maybe not the sex, I'm not sure)